Along with the improvement of people's living standards, and because of the diet not quite reasonable, the number of people suffering from obesity has been increasing for years. Because it may trigger or aggravate diabetes, fatty liver, and cardiovascular system diseases, etc., obesity has become one of the serious diseases threatening human health. A weight-reducing aid, Orlistat, developed by Roche, Switzerland, is a non-systemic active and specific gastric and pancreatic lipase inhibitor with long and potent activity, which may inactivate the lipases in the gastrointestinal tract by covalently bonding the active serine residues, thus blocking the hydrolysis and absorption of about 30% fat in food, which results in body weight loss. Researches demonstrate, while long-term weight control in obese patients, this drug can reduce incidence of hyperlipemia, hypertension and hyperglycemia as well as other obesity-related diseases. Orlistat has advantages of little systemic absorption, non-accumulation during repeated administration, low serum concentration, and non-tolerance etc, while its side effects are mainly of gastrointestinal reactions. As the first OTC weight-reducing aid approved by U.S. Food and Drug Administration (FDA), Orlistat has been marketed in over 100 countries. Currently about 8 million people worldwide are taking Orlistat, which ranks the number one sales of weight-reducing drugs in the worldwide market. Therefore, there is a promising future for the development of Orlistat. At present, API of Orlistat products used in clinic is mainly prepared by reducing natural lipstatin which is used as raw material, but the natural lipstatin is not readily available so that Orlistat prepared by this method is of high production cost, and the patients are not able to afford it. Thus, the preparation of Orlistat by total synthesis process helps reduce the cost of medication for patients. By analyzing the structure of Orlistat, it can be seen that the β-lactone intermediate is an important intermediate for the synthesis of Orlistat, and the target product Orlistat can be made from this intermediate through Mitsunobu reaction. Therefore, the development of a preparation method of β-lactone intermediate which has short synthetic route and low-cost, suitable for industrial production, would bring important prospect and economic value.
The β-lactone intermediate mentioned in the present invention is (3S,4S)-3-hexyl-4-((R)-2-hydroxytridecyl)-oxetan-2-one, its structure formula represented by formula (I):

The reaction equation of preparing Orlistat from β-lactone intermediate is as follows:
wherein, 3-situs and 4-situs of β-lactone intermediate has been labeled.
The literature search shows that there are mainly three methods for construction of the β-lactone intermediate: (1) constructing the β-lactone intermediate through condensation and cyclization of alkyl-aldehyde with alkenyl ether or silyl ketene having hexyl side chain in the presence of Lewis acid as catalyst; (2) firstly synthesizing a β-hydroxyl acid through Aldol condensation reaction of alkyl aldehyde with n-octanoic acid ester or through tri-substituted six-membered lactone intermediate or its analogue as key intermediate, then synthesizing the β-lactone intermediate through the lactonization of the β-hydroxyl acid; and (3) directly constructing the β-lactone intermediate through photocatalytic alkylation using alkenyl lactone as raw material. Among the above three methods, if compared with the method (2), the method (1) needs less reaction steps and is more favorable for reducing industrial production costs thus having obvious cost advantage; although the method (3) has less steps, it needs expensive reagents and specific reaction environments for photocatalytic alkylation, not favorable for the realization of industrial production. It can be seen that at present the method (1) is a more favorable synthesis route to reduce the production cost. The synthesis routes of the three methods mentioned above are as follows:

In the above three methods, R represents a hydroxyl protecting group.
As seen from the main synthesis methods of β-lactone intermediate above mentioned, in the method (1), alkyl-aldehyde is a key precursor compound for the total synthesis of Orlistat. Currently the commonest synthesis method of alkyl-aldehyde is a one-step reaction of reducing alkyl acid ester using DIBAL reagent at a low temperature of −78° C. to obtain the aldehyde, and the β-hydroxyl protecting group used therein is usually benzyl, tetrahydropyranyl, tert-butyldimethylsilyl and the like. However, the reductive reagents DIBAL used in this method are relatively expensive and the low reaction temperature required is a harsh condition, so that the method has low economic value and is not suitable for large scale production in industry. The method of construction of β-lactone intermediate using reagent DIBAL is as follows:

wherein, R represents a hydroxyl protecting group.